1. Field of the Invention
This invention relates to an apparatus for controlling the temperature of an oxygen sensor, which is mounted on an exhaust pipe to control an air/fuel ratio of an internal combustion engine, and more particularly to an apparatus for controlling activation of a heater of the oxygen sensor by turning off the electricity to the heater when the temperature of the exhaust pipe, when the engine is started is low, or when condensation left in the exhaust pipe or is likely to occur in the exhaust pipe.
2. Description of Background Art
A conventional oxygen sensor has varying conductivity and electromotive force if there is a difference in oxygen concentration between confronting surfaces of two electrodes of the oxygen sensor, which are arranged with a solid electrolyte interposed therebetween.
Using the oxygen sensor, a difference in concentration between oxygen contained in exhaust gas in an exhaust pipe and the oxygen in the atmosphere is detected in terms of internal resistance variation in conductivity and/or electromotive force. The variation of the detected value is then provided to a control unit to control a fuel injection time period in such a manner that the intake mixed gas achieves a theoretical air/fuel ratio.
The term xe2x80x9ctheoretical air/fuel ratioxe2x80x9d is a ratio of air and fuel by weight for a theoretically required minimum amount of air, under the hypothesis that mixed gas introduced into a combustion chamber can be burned perfectly.
In a gasoline engine, the typical theoretical air/fuel ratio has an air component which is in the order of 14.5-15, and a fuel component which is in the order of 1.
Controllers are known which detect a temperature of the exhaust pipe at a time of starting an internal combustion engine. These controllers use a temperature sensor in the exhaust pipe, then detect an element temperature of an oxygen sensor by an element temperature detector, and control a heater of the oxygen sensor in such a manner that the oxygen sensor temperature does not exceed a predetermined temperature until the exhaust pipe temperature detected by the temperature sensor has exceeded 100xc2x0 C. (For example, see Japanese Utility Model Laid-Open Publication No. Hei 5-84852).
Furthermore, controllers have been proposed in which an internal resistance of an oxygen sensor is estimated. This approach is used because, as is commonly known, an oxygen sensor operates stably when a high temperature, an internal resistance value and an oxygen sensor temperature correlate. The heater is controlled in such a manner that the oxygen temperature is kept normally constant within a high temperature range exceeding a predetermined temperature. (For example, see Japanese Patent Publication No. Hei 7-99365 and Japanese Patent Laid-Open Publication No. Sho 59-214756).
However, the conventional electricity controllers require a temperature sensor for detecting that an exhaust pipe temperature is sufficiently high enough to eliminate condensation. The temperature effectively prevents thermal shock to the oxygen sensor thereof immediately after the internal combustion engine has been started; however, such a design has increased the cost of production.
Another problem with the conventional controllers is that the method of either assembling or mounting the temperature sensor is complex, thereby ensuring large production cost requirements and requiring significant cost when maintenance is required.
As indicated with the conventional controllers for estimating an internal resistance of the oxygen sensor, it is possible to keep the temperature of the oxygen sensor normally constant at an optimum temperature by way of a heater. This keeps the internal resistance value of the oxygen sensor constant. However, these conventional controllers encounter still another problem. Although a temperature of the oxygen sensor can be determined from an internal resistance thereof, it can not estimate, for an exhaust pipe in question, a temperature when condensation will develop in the exhaust pipe.
With the foregoing problems in view, it is therefore an object of this invention to provide an inexpensive apparatus for controlling the timing of turning on and off the electricity to an oxygen sensor heater in order to prevent thermal shock, without using an exhaust pipe temperature sensor, even when condensation develops in the exhaust pipe because the temperature of the exhaust pipe, which communicates with the internal combustion engine, is low.
In order to attain the above-mentioned object, according the present invention, there is provided an apparatus for controlling the timing of turning on the electricity to a heater by controlling the temperature of an oxygen sensor, which is mounted on an exhaust pipe to control an air/fuel ratio of an internal combustion engine, comprising: internal resistance detecting means for detecting an internal resistance value of the oxygen sensor; and controlling means for estimating a surface temperature of the oxygen sensor based on the internal resistance value detected by the internal resistance detecting means, then estimating a temperature of the exhaust pipe based on the estimated surface temperature of the oxygen sensor and controlling the starting of turning on the electricity to the heater, which is combined with the oxygen sensor in a single unit, when the estimated temperature of the exhaust pipe has reached a predetermined temperature.
With this arrangement, since there are provided internal resistance detecting means for detecting an internal resistance value of the oxygen sensor and controlling means for estimating a surface temperature of the oxygen sensor based on the internal resistance value detected by the internal resistance detecting means, then estimating a temperature of the exhaust pipe based on the estimated surface temperature of the oxygen sensor and starting turning on the electricity to a heater, which is combined with the oxygen sensor into a single unit, when the estimated exhaust pipe temperature has reached a predetermined temperature, it is possible to prevent turning on the electricity to the heater while condensation remains in the exhaust pipe because the exhaust pipe temperature is low and until after such condensation has disappeared. Thus, it is possible to control the timing of turning on the electricity to the heater with a simple structure.
As a preferred feature, the heater control apparatus further comprises an intake temperature sensor for detecting an intake temperature. Thus, the controlling means controls the timing of turning on the electricity to the heater based on the estimated exhaust pipe temperature and the detected intake temperature.
With this preferred arrangement, partly since the intake temperature sensor is provided for detecting an intake temperature and partly since the controlling means controls the starting of turning on the electricity to the heater based on the estimated exhaust pipe temperature and the detected intake temperature, it is possible to heat the oxygen sensor when no condensation remains in the exhaust pipe.
As another preferred feature, the heater control apparatus further comprises an engine water temperature sensor for detecting an engine water temperature. Thus, the controlling means controls the timing of turning on the electricity to the heater based on the estimated exhaust pipe temperature and the detected engine water temperature.
With another preferred arrangement, partly since the engine water temperature sensor is provided for detecting an engine water temperature, and partly since the controlling means controls turning on the electricity to the heater based on the estimated exhaust pipe temperature and the detected engine water temperature, it is possible to heat the oxygen sensor when no condensation is in the exhaust pipe.
As still another preferred feature, the heater control apparatus further comprises an intake temperature sensor and an engine water temperature sensor. The controlling means detects an intake temperature and an engine water temperature and controls the timing of turning on the electricity to the heater based on the estimated exhaust pipe temperature, the detected intake temperature and the detected engine water temperature.
With still another preferred arrangement, partly since the intake temperature sensor and the engine water temperature sensor are provided, and partly since the controlling means controls the timing of turning on the electricity to the heater based on the estimated exhaust pipe temperature, the detected intake temperature and the detected engine water temperature, it is possible to heat the oxygen sensor when no condensation remains in the exhaust pipe.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.